Battery explosion attenuation material and method

ABSTRACT

A porous compressible plastic material is inserted into the head space above the cells in a storage battery to attenuate the explosion of combustible gases accumulating therein. The attenuation material has a unique bimodal pore distribution, including a major proportion of small pores effective in attenuation and a minor proportion of large pores effective in gas and electrolyte management. Both open cell and fibrous materials may be utilized.

BACKGROUND OF THE INVENTION

The present invention relates to electric storage batteries or cellsand, in particular, to an improved material and method effective inattenuating an explosion of combustible gases which accummulate in thehead space of electric storage batteries.

As is well-known in the art, most types of electric storage batteriesgenerate combustible gases during operation, which gases are eithervented from the battery container into the atmosphere or are recombinedwithin the battery in secondary reactions with the active materials.However, even in battery constructions which are intended to provide forthe internal recombination of combustible gases, there are certaincircumstances, such as inadvertent or abusive overcharge, where therecombination mechanism is ineffective and significant volumes ofcombustible gases will be generated.

It is also well-known that the combustible gases within the head spaceof a battery may be accidentally ignited and result in explosion of thebattery. The damage and injury resulting from such explosions are welldocumented. Thus, for many years, effective and reliable means have beensought for preventing or minimizing explosions in batteries and thehazardous effects thereof.

The ignition of combustible gases within the head space of a battery canbe caused by either an internal or external ignition source. Combustiblegases which are generated within a battery, if not effectivelyrecombined, will eventually create a high enough internal pressure sothey must be vented to the atmosphere. The venting is typicallyaccomplished through the use of a simple open vent slot or a one-wayrelief valve, sometimes referred to as a "burp" valve. During venting ofcombustible gases an external source of ignition, such as a flame orspark near the battery vent, can result in an ignition which willpropagate back into the battery container and result in an explosion.However, improvements in relief valve construction and the developmentof flame arrestors which are used in conjunction with vents havedecreased considerably the incidence of battery explosions caused by anexternal ignition source, provided such protective devices have not beenremoved or disabled, or the integrity of the container or coverotherwise breached.

However, should an external source of ignition breach one of theprotective devices or should an ignition occur within the container, thecombustible gases in the head space may explode. The concentration ofgases, typically a mixture of hydrogen and oxygen, and the relativelylarge volume of the head space can result in an explosion which willshatter the container, cover or other components. In addition, theexplosion will also often carry with it the liquid acid or otherhazardous electrolyte from within the container.

Thus, it is not surprising that materials and methods for suppressing orminimizing the effects of explosions within batteries have been longsought. It is, of course, axiomatic that elimination of the open headspace or substantially filling the head space with a solid materialwould virtually eliminate the possibility of an explosion simply becausethe presence of combustible gases would be eliminated. However, neitheralternative is acceptable. An open head space is necessary in virtuallyall secondary storage batteries. First of all, the head spaceaccommodates certain battery components, such as plate straps, intercellconnectors, or terminals. In addition, in batteries which utilize freeliquid electrolyte, sometimes referred to as "flooded" systems, openhead space is necessary to accommodate variations in the level of theelectrolyte as the battery is cycled or to provide space for acidmovement under extreme conditions of use, such as abusive overcharge.Also, the head space accommodates movement of the electrolyte level asthe battery is tilted in service, such as the ability to operate anautomobile on an incline without loss of electrolyte.

For many years, it has been known to fill the head space in a battery orcell, either partially or totally, with a porous material to inhibit theexplosion of gases within the head space and quench any flame which maybe formed, while still allowing the movement of gases and electrolytethrough the material. For example, U.S. Pat. No. 2,341,382 disclosespartially filling the head space with a loosely packed material, such ascrushed stone or glass, diatomaceous earth, or glass wool. Thedisclosure in that patent suggests that the loosely packed fillermaterial will not prevent the explosion of gases entirely, but bydividing the head space chamber into many minute interconnected cells, arapid total combustion of the gases is prevented and, instead, a seriesof weak and inconsequential minor explosions will occur until the flameis quenched. It is believed that the general theory set forth in thatpatent, sometimes called the "chain termination" theory, is essentiallycorrect and valid for a large variety of porous filler materials.However, notwithstanding the soundness of the theory and the developmentin the ensuing years of many improved porous materials, particularlyplastics, there has been no large scale or general implementation of thetechnology. Thus, there still exists in the battery industry today aserious need for a material and method of utilizing it which willeffectively attenuate hazardous explosions, but will otherwise not bedetrimental to safe and efficient operation of the battery.

There are a number of factors which are believed to have generallyinhibited or prevented the practical and useful application of explosionsuppression or attenuation technology in batteries. Broadly, thesefactors include the creation of other hazards and detrimental effects onbattery performance. As the head space of a battery is filled with aporous material, there will be a decrease in the actual remaining voidvolume in the head space inversely proportional to the porosity oreffective void volume of the filler material. In other words, the moresolids present in the filler material, the greater will be the reductionin the total head space volume filled with such material. As indicatedabove and particularly in flooded batteries, the loss of actual openhead space volume will lessen the space available for electrolytemovement or electrolyte level variations.

It is known that high rate charging or excessive over-charge can resultin vigorous gassing in many types of batteries. If the gas bubblesformed in the electrolyte cannot find ready and fairly direct channelsto the battery vent openings, electrolyte may be upwardly displaced andoverflow through the battery vents. This condition is known aselectrolyte "pumping" and the damaging and hazardous effects of acorrosive electrolyte flowing out of a battery are obvious.

Electrolyte pumping can also occur even where the head space of thebattery is filled with a very highly porous material, i.e. a materialhaving a high void volume. For example, an open cell foam material mayhave a void volume as high as 97 to 98% and, if placed in the head spaceof a battery, will only occupy about 2 or 3% of the total volumethereof. Nevertheless, in a flooded battery, such a material may readilyretain electrolyte and not allow it to drain back into the battery bygravity. Electrolyte so retained in a porous filler material will bereadily pumped from the battery under the conditions of vigorousgassing, described above.

In addition, if a relatively large volume of electrolyte is drawn fromthe cells through wicking by a porous material in the head space or ifthe porous material otherwise retains the electrolyte with which itcomes into contact, insufficient electrolyte may remain in the cells forproper electrochemical reaction and operation of the battery. Also, anymaterial to be used as an attenuation material in batteries must possesscertain other critically necessary physical properties. Such materialsmust have adequate resilience to retain their shape and to readily fillsometimes irregular shape of battery head space. The material must alsobe thermally and chemically stable in the operating environment withinthe battery. To provide adequate safety, any attenuation material mustbe able to survive repeated ignitions without melting or sintering. Amaterial capable of effectively operating only once, but being destroyedin the process, would not be satisfactory. The material cannot, ofcourse, dissolve in or otherwise react with the liquid electrolyte.

A number of porous plastic materials have been used in fuel tanks orsimilar containers as a means for reducing the explosion hazards. Bothfibrous and cellular plastics of various kinds are disclosed in the art.U.S. Pat. No. 3,561,639 discloses the use of a single block of open cellpolyurethane foam to fill the interior of a fuel tank. The describedmaterial has a reticulated or fully open pore structure, a pore sizeranging from 10 to 100 pores per linear inch (ppi), and a void volume of97%. The fully reticulated structure is described as important to keepflame propagation from reaching the velocity necessary for explosion andto provide a high degree of permiability for the liquid fuel. A materialused today for explosion safety in jet aircraft fuel tanks is anether-base polyurethane foam having a pore size of 20 ppi which isproduced by Scott Foam and sold under the name "Protectair".

Bulked fibrous plastic materials of many types have also been proposedfor use as a means of arresting flames and reducing explosion hazards infuel tanks. The filamentary plastic materials proposed for such useinclude polyolefins, nylon, dacron, polyesters, acrylics, andpolyurethanes, as well as others. The materials are typically bulked ortextured to provide high porosity and void volume by any of manywell-known methods such as twisting, looping, crimping, needle punchingand so forth. Examples of various types of such materials are describedin U.S. Pat. Nos. 3,650,431, 4,141,460, and 4,154,357.

Notwithstanding the broad use of the foregoing porous plastic materialsto suppress explosions in fuel tanks, we are unaware of any effectiveuse of these materials in storage batteries and, in particular, as anexplosion attenuation material in the open head space of such batteries.As a result of extensive testing, we have found as a general matter thatthe materials which perform most effectively to attenuate an explosionand quench the flame resulting from the ignition of combustible gases,do not perform well in other aspects of battery operation. As previouslyindicated, the violence of an explosion (in terms of the peak pressuredeveloped within the open head space of a battery) can be reduced bysubstantially filling the head space with a porous material. Smallspaces not effectively filled with the porous material and within whichcombustible gases are ignited will result in minor pressureperturbations. Certain porous materials will attenuate the violence ofthe explosion and eventually quench the flame. We have found that thepressure developed during an explosion is reduced as the pore size ofthe attenuation material is decreased. Unfortunately, as the pore sizeof the material decreases, the adverse effects of the material onbattery performance increase. The smaller the pore size of the material,the greater the propensity of the material to wick up electrolyte or toretain within the pores electrolyte with which it is wetted. Electrolytewhich is retained in the pores and cannot drain back into the cell canresult in two serious problems, as previously mentioned. First,electrolyte retained in the porous material is not readily available forelectrochemical reaction and may thus result in diminished electricalperformance. Retained electrolyte will also inhibit the flow of gasesgenerated within the battery and, in certain circumstances of operation,result in electrolyte being pumped out of the battery through the ventopenings.

SUMMARY OF THE INVENTION

The present invention is directed to an improved porous plastic materialwhich has a unique bimodal pore distribution including a majorproportion of the small pores most effective in explosion attenuationand a minor proportion of large pores which are required to accommodategas and/or electrolyte movement within and through the head space duringbattery operation. The unique bimodal function may be provided by usinga single porous material, properly prepared and installed, or by using acomposite of two different porous materials. Furthermore eitherfilamentary or open cellular materials may be used.

In one embodiment of the invention, which has been found to beparticularly suitable for use in secondary alkaline systems, a singlematerial comprising small pillows of lofted, non-woven polypropylenefibers is used to fill the open head space in each battery cell. Thematerial in each of the pillows provides the small pore or microporousstructure effective in the attenuation of explosions and the randomorientation of the numerous small pillows used to fill the head spaceresults in the large pore or macroporous structure required for propergas and electrolyte management. By packing the polypropylene fiberpillows rather loosely within the head space or with only a slightamount of compression, the macroporous structure between adjacentrandomly oriented pillows provides adequate open space or channels orchannels for the venting of gases generated within the battery and thepassage or drain back of any mobile electrolyte.

A compressible open cell plastic material may also be used to provide asimilar bimodal pore function. Polyurethane foam material may be cut orchopped into small, and preferably uniformly shaped, pieces with whichthe head space of the cell is filled in a somewhat loosely packedfashion. The bimodal pore function is provided in a manner similar tothe polypropylene fiber material previously described. Thus, the smallpore open cell structure of the foam provides the attenuation/quenchingfunction and the spaces or large pores between the pieces provide theopen spaces for gas and electrolyte movement.

In a preferred embodiment of the invention, the bimodal poredistribution is provided by a composite of two different porous,compressible plastic materials. A composite of two different types ofopen cell polyurethane foam has been found to be particularly effective.The small pore component of the composite material comprises anon-reticulated polyurethane foam with a pore size of 60 ppi. The largepore component of the composite comprises a reticulated polyurethanewith a pore size of 20 ppi. The composite is a blend of uniform smallpieces of the two materials, preferably having a ratio of small pore tolarge pore pieces of about 3 to 1.

The composite bimodal material of the preferred embodiment is preferablyinstalled in the open head space of a battery cell so that it isretained therein in a compressed state, e.g. about 20% compression.Maintaining some compression on the composite foam provides two separatebenefits. If the material is maintained in compression within the headspace, the inherent resilience of the polyurethane foam will tend tocause it to fill the entire open volume of the head space before it hasreached it free, fully expanded state. This helps assure there will beno significant open volumes within the head space which would allow anexplosion of more than minor and insignificant proportions to occur. Inaddition, it has been shown that open cell foams maintained incompression attenuate explosions better (result in lower peak pressures)than the same material in a free, uncompressed state. The compositebimodal polyurethane foam material is particularly effective as anexplosion attenuation material in flooded lead-acid batteries.

The materials providing the unique bimodal function disclosed herein,whether of single or composite construction, provide the capability tosafely attenuate explosions in a wide variety of storage batterieswithout inhibiting their electrical performance or creating additionaloperational hazards. Various materials may be selected and optimized foruse in either acid or alkaline electrolyte systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a storage battery with portions of thecover and container broken away to show a typical loading of theattenuation material of the present invention in the head space of thebattery cells.

FIG. 2 is an enlarged sectional view taken on line 2--2 of FIG. 1.

FIG. 3 is an enlarged top plan view of the attenuation material in FIG.1, showing the preferred bimodal composite construction utilizing opencell foam material.

FIG. 4 is an enlarged sectional view of the bimodal material ofcomposite construction taken on line 4--4 of FIG. 3.

FIG. 5 shows the composite bimodal attenuation material packaged forinsertion in the head space of a cell.

FIG. 6 is an enlarged sectional view taken along line 6--6 of FIG. 5.

FIG. 7 shows an alternate embodiment of an attenuation material madefrom a single type of bulked filamentary plastic.

FIG. 8 is an enlarged sectional view taken on line 8--8 of FIG. 7.

FIG. 9 is an enlarged perspective view of a single pillow of theattenuation material shown in FIGS. 7 and 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a storage battery 10 includes the container 12 andcover 14. The container and cover are typically made of injection moldedpolypropylene and have average wall thicknesses of about 0.1 inch. Thecontainer 12 is divided into a series of cells by integrally moldedintercell partitions 16. Each cell contains an electrode element 18constructed of a stack of alternating positive and negative platesspaced apart by insulating separators, all in a manner well known in theart. The electrode elements 18 within each cell also include the typicallug and strap connectors, not shown, and adjacent electrodes are seriesconnected through the cell partition 16 with appropriate conductiveintercell connectors, also not shown, but comprising conventionalconstructions well known in the art. The end cells of the battery 10also include connections to the exterior terminals 20 through thecontainer wall, as shown, or through the cover 14 in the case of topterminals.

The cover 14 includes a series of vent/fill holes 22, one for each cell.The vent/fill holes 22 are closed with vent cap assemblies 24 which maybe fixed or removable. The vent cap assemblies 24 typically includedownward depending vent plugs which substantially seal the holes 22, butare provided with small holes or passages to allow gases generatedwithin the cell to vent to the atmosphere. The vent assembly 24 alsotypically includes a microporous flame arrester adjacent each hole 22through which gases may vent, but which is intended to prevent anexternal flame or ignition source from propagating back into the cell ofthe battery. The foregoing construction is typical and the variouselements need not be shown for an understanding of the invention.

In the assembly of a typical lead-acid battery of the foregoingconstruction, the assembled electrode elements 18 are placed in thecells of the battery, the intercell connections between adjacentelectrode elements are made through the partitions 16 (and, depending onthe type, the terminal connections may also be made), and the cover 14is sealed to the container 12. Each of the cells is filled with asulfuric acid electrolyte to a level slightly above the tops of theelectrode elements 18 and the battery is formed by electochemicallyconverting lead oxide material in the positive and negative plates tolead dioxide and lead, respectively.

Except for the space occupied by the plate straps, intercell connectorsand terminal assemblies, previously mentioned, the space within eachcell above the top of the electrode element 18 and below the undersideof cover 14 is generally open. This open head space, designatedgenerally by the letter H in FIGS. 1 and 2, but shown occupied by theattenuation material to be hereinafter described, may have a volume of20 to 25 cubic inches (325 to 410 cubic centimeters) per cell in atypical 6-cell 12 volt battery.

The hydrogen and oxygen gases, which are generated as a result of theelectrochemical reactions within the cell, pass upwardly through theelectrolyte, between the plates and separators of the electrode element18 and accumulate in the head space H, until a sufficient positivepressure is established to cause the gases to vent through the ventholes 22 and past the flame arrestors or other venting construction inthe vent caps 24. The gas mixture is, of course, highly explosive and,as is well known, an ignition of such gases accumulating in the openhead space will result in an explosion which can easily shatter thecontainer and/or cover, as well as other elements connected thereto. Inaddition to destroying the battery, the potential personal danger fromexploding battery pieces and acid electrolyte is well known. Because ofthe need to accommodate certain structural components of the battery andto provide space for electrolyte level fluctuations, the head space inbatteries must be maintained.

In the preferred embodiment of the present invention, the bimodal poredistribution which is necessary for effective attenuation andgas/electrolyte management, is provided in a composite of two differenttypes of open cell polyurethane foam materials. Referring also to FIGS.3 and 4, the composite attenuation material 26 is comprised of small,randomly oriented small pore pieces 28 and large pore pieces 30. Thesmall pore pieces 28 are most effective in the attenuation of explosionsand the large pore pieces 30 provide the open space for back-flow of theelectrolyte and movement of the exiting gases to the vent holes 22.

One blend of foam materials which has been found to provide aparticularly effective composite comprises 75% small pore pieces 28 ofScott Foam C100-30 and 25% large pore pieces 30 of Scott Foam"Protectair". Both materials are ether-based polyurethane foams,however, the small pore material is non-reticulated and the large porematerial is fully reticulated. The basic distinction between the two isthat, in non-reticulated foam, not all of the cells are fully openalthough the material is permiable. In a reticulated foam, the cellmembranes or bubbles are completely broken or open, resulting in higherpermiability. The small pore pieces have a pore size of 60 pores perinch (ppi) and the large pore pieces have a pore size of 20 ppi.

The composite bimodal pore material 26 may utilize cut or chopped piecesof the foam materials 28 and 30 of any convenient size or shape. Thecomposite material, in a random distribution of pieces, may be loadedinto the open head space H of each battery cell in any convenient mannerwhich will assure that the head space is substantially completely filledwith the material 26. These polyurethane foams are highly compressibleand resilient and, therefore, inserting the material into the head spaceunder some precompression and allowing it to expand within the headspace will help assure complete filling. Using smaller pieces 28 and 30also enhances complete filling. Foam pieces nominally 1/2 inch in sizehave been found to work satisfactorily.

There are a number of methods and related apparatus which have beenfound to be particularly unique and useful in preparing these explosionattenuation materials for and loading them into the battery cells. Someof these methods and apparatus will be described briefly hereinafterand, with other alternative methods, are more fully disclosed andclaimed in the commonly-owned copending application of Binder et al,entitled "Method and Apparatus for the Preparation and Installation ofBattery Explosion Attenuation Material".

It has been found that by maintaining the composite material 26 in astate of compression within the head space ranging, preferably, from 5to 20%, substantially complete filling of the head space and optimumbimodal performance are attained. The small pore pieces 28, with a poresize of 60 ppi, operate to attenuate explosions very effectively in anuncompressed free state, but are even more effective when maintained incompression. A study of the attenuation mechanism suggests that thesmall pore open cell foam inhibits and prevents the complete reaction ofthe total volume of gases present in the head space. The multiple opencell paths provided in the foam material slow the rate of ignition andexplosion and eventually act to quench the flame before completecombustion of the gases has occurred. Small, inconsequential explosionsor "pops" have been seen to occur in the portions of the head space notcompletely filled with the porous material. The pressure developed bythese minor explosions, however, is relatively insignificant and, if theattenuation material is properly prepared and installed, such explosionshave been found to cause no damage to the battery container, cover, orthe internal battery structure.

The large pore pieces 30 of the composite material 26 provide the neededopen pore space to allow the generated gases to pass relativelyuninhibited from the electrode elements 18, through the attenuationmaterial 26, and out the vent openings 22. Gas movement may be furtherinhibited by liquid electrolyte retained in the pores of the attenuationmaterial and effectively blocking the paths available for gas movement.The large pore material 30 does not tend to retain acid and allows theacid to drain back readily into the cell. This is especially importantto prevent electrolyte from being pumped out of the battery through thevent openings, especially when a battery is being charged. If the poresof the attenuation material are filled with electrolyte, vigorousgassing which occurs, for example, during high rate overcharge, willpump acid from the battery. Thus, the large pore material provides thenecessary drainback of acid electrolyte and open paths for flow ofgases. The open and relatively unrestricted flow paths provided by thelarge pore material 30 is also important to allow safe and relativelyunrestricted filling of the cells with electrolyte after initialassembly of the battery.

The dispersion of large pore pieces 30 both horizontally (FIG. 3) andvertically (FIG. 4) throughout the composite material 26 is intended toinsure that there are a sufficient number of completely openinterconnected large pore channels. On the other hand, it is desireableto keep the percentage of large pore material at a minimum since it isless effective than the small pore material in attenuating explosions.However, it is believed that fairly wide variations may be made in thecomponents of the composite material 26 while still providing effectiveattenuation and electrolyte/gas management. Thus, changes in the poresizes, ratios of the two component materials, relative sizes of thepieces, and the compression with which the material is held in place maybe made.

Various methods may be used to prepare the composite attenuationmaterial for loading into the head space of a battery and to facilitateits loading and proper ultimate positioning therein. Referring to FIGS.5 and 6, an appropriate blend of large and small pore foam pieces may becompressed and rolled into a cylindrical shape and retained incompression by a mesh or net 32 of suitable material, such as nylon. Acylinder 31 of compressed composite material is placed in the head spaceof each cell over the electrode element 18 prior to placing the cover 14on the container 12. The nylon net 32 is soluable in sulfuric acid andwill dissolve in a short time after the battery is filled withelectrolyte. Dissolution of the net will result in expansion of thecompressed composite material to completely fill the head space. Byappropriately controlling the size of the compressed cylinder 31 and thedegree of compression of the porous material, in consideration of thesize of the head space to be filled, substantial variations in thecompression with which the material is ultimately held in the head spacemay be attained.

FIGS. 7 through 9 show an alternate embodiment of an attenuationmaterial utilizing only a single type of porous compressible plastic.The material is nevertheless able to provide the same bimodal functionas the composite material of the preferred embodiment. The materialcomprises randomly oriented pillows 34 of lofted non-woven polypropylenefibers 36 which are placed within and fill the battery cell head spacein a manner similar to the composite material of the preferredembodiment. One type of lofted or bulked polypropylene material whichhas been found to be effective is made of 15 denier fibers and, in a mat0.56 inch thick, weighs 6.2 oz./sq. yd.

However, the pillows 34 are less tightly compressed and, in their finalorientation within the head space, provide a bimodal pore distributionin a somewhat different manner. The small pore or microporousdistribution effective to attenuate explosions is provided by the bulkedfibrous material. The large macropore distribution needed to accommodatethe flow of gases and electrolyte through the material is provided bythe open spaces or channels 40 between the randomly oriented pillows 34.

When using a single type of porous material and relying on the spacingbetween the pieces to provide the desired macroporosity, it is importantthat the material not be too tightly compressed so that the channels 40are eliminated. However, in order to facilitate handling and loading ofthe material, it is still desirable and possible to enclose the randomlyoriented pillows 34 within a net 38 and under some slight amount ofcompression. Upon ultimate dissolution of the net 38 by the electrolytewithin the cell (in the same manner previously described with respect tothe composite material shown in FIGS. 5 and 6), the material may beallowed to expand to its essentially free state to provide adequatelarge pore channels 40. Nevertheless, the initial compression with whichthe material is bound will enhance handleability and loading into thecells and the subsequent expansion in situ to substantially fill thehead space. Also, choosing an initial compression and/or amount ofmaterial which will result in retaining some level of compression in thematerial after expansion in the head space serves to insure against lossof fill through settling or compaction of the material.

The fibrous polypropylene has been found to be particularly well suitedfor use in a wide variety of flooded secondary systems, both acid andalkaline. Polypropylene is, of course, stable and essentially insoluablein aqueous sulfuric acid solutions used in lead-acid batteries. Inflooded alkaline systems, typically utilizing an aqueous potassiumhydroxide electrolyte, polypropylene is the only useful and commonlyavailable plastic material which will not dissolve or degrade in thealkaline electrolyte or sinter or melt in the presence of a flame. Thus,although a fairly wide variety of cellular and fibrous plastics appearto be suitable for use as attenuation material in lead-acid batteries,polypropylene appears to be the only material, practically suited foruse in alkaline systems, considering effectiveness, cost and stability.

The bimodal function, which may be provided with a single type ofmaterial such as the polypropylene fibers described above, may also beprovided by other plastic materials of a single type. For example, thesmall pore pieces 28 of open cell polyurethane foam are also effectivein providing the necessary bimodal function, if the pieces are properlyprepared, sized and installed. For example, filling the head space withrandomly oriented cubes of polyurethane foam having a nominal edgedimension of 1/2 inch (in a substantially uncompressed state), willprovide effective attenuation, and the open spaces or channels betweenthe cubes will provide the large macropore function necessary foreffective gas/electrolyte management. In this manner, the cubed opencell polyurethane foam operates in a manner similar to the fibrouspolypropylene pillows 34 described with respect to the embodiment shownin FIGS. 7 through 9. The cube shape of the polyurethane foam pieces,when used as the sole type of attenuation material, has been found towork better than pieces of other shapes to provide the dual large/smallpore function. Cube shaped pieces tend to assume and maintain a betterrandom orientation, thereby assuring adequate open channels between thepieces for the gas/electrolyte management function. Other shapes of foammaterials, such as chopped, shreadded or other random sizes will tend toorder themselves or pack more tightly and not provide the optimum openchannel structure.

The use of the foregoing materials in both the open cell foam and bulkedfiber forms have been shown to provide another benefit when used to fillthe head space in a battery. The materials have been found to be veryeffective in preventing the evolution of electrolyte mist from thebattery vents 22 when vigorous gasing occurs. This phenomenon issometimes referred to as electrolyte "spewing" and is simply the upwardand outward movement of atomized electrolyte created by the breaking ofgas bubbles. Spewing can occur, for example, during initial formation oron overcharge. Electrolyte spewing is minimized or eliminated becausethe fibrous or cellular materials filter the atomized mist from theevolving gases. In some flooded systems which are known to gasparticularly vigorously, such as alkaline nickel-iron cells, theattenuation material will also filter active material oxides which arespewed from the electrodes, as well as the electrolyte mist. Spewing, ofcourse, is detrimental from the standpoint of electrolyte loss, as wellas the corrosive and other hazardous effects of its venting to theatmosphere.

The explosion attenuation materials disclosed herein have been describedas being particularly useful in batteries having flooded electrolytesystems. However, there are a variety of secondary storage batterieswhich operate with an immobilized gelled electrolyte or with a so-called"starved" electrolyte system. In either of these non-flooded systems,the intent is to eliminate free liquid electrolyte within the cells suchthat the battery can be operated in any attitude without electrolyteloss.

Certain starved electrolyte systems also are constructed to operate onthe principle of oxygen recombination such that, during normaloperation, evolved oxygen is recombined within the active material ofthe electrodes and does not accumulate within the battery or have to bevented to the atmosphere. Typically, such batteries are also made withan excess of negative active material in an attempt to prevent thegeneration of hydrogen gas.

In gelled or starved electrolyte systems, although an open head space isnot ordinarily required for the gas/electrolyte management needed inflooded systems, an open head space is still required to accommodate thecell elements which are similar in batteries of all kinds. Also,notwithstanding the intent in starved or gelled systems to eliminate orreduce the generation of potentially explosive gases, such gases areoccasionally evolved in these systems and accumulate, prior to venting,in the open head space. Thus, during formation of these batteries orduring unusual circumstances of use, such as inadvertant or abusiveovercharge, both hydrogen and oxygen gas may be evolved. Indeed,batteries which utilize a gelled acid electrolyte, oxygen recombinationis typically slower than in true recombination systems and hydrogen isalso more readily generated even under open circuit conditions. Thus,there is direct applicability of the explosion attenuation conceptsdescribed herein in non-flooded systems as well.

In addition, both starved and gelled electrolyte batteries are ofteninitially formed with an ordinary aqueous acid electrolyte. Theformation electrolyte is removed or dumped after formation and replacedwith a substantially diminished amount of an immobilized operatingelectrolyte. During formation, however, the same gas/electrolytemanagement concerns exist as in flooded systems and a bimodalattenuation material provides the same useful benefits. However, as theamount of liquid electrolyte needed to be handled (i.e. present orpotentially present in the head space) decreases, the percentage oflarge pore material needed would also decrease. Of course, if there isnever a need to handle free liquid electrolyte in the head space of abattery cell, essentially only the small pore material needed forexplosion attenuation would be required.

In addition to the physical properties of materials thus for describedwhich contribute directly to the important bimodal functioning, thereare a number of other important characteristics which a material shouldpossess to optimize its utility. The material should have a large totalvoid volume such that it does not occupy too much of the total volume ofthe headspace. The various materials described herein all possess porevolumes well in excess of 90% and, therefore, result in little loss ofnatural headspace volume. The material must also have good resilience toretain or return to its original shape after compressive distortion.Such resilience is also characteristics of the materials described. Thematerials must also possess thermal stability against degradation in theface of the high temperatures potentially encountered in use. Inparticular, the material should be capable of attenuating explosions andquenching a propogating flame front in a dry atmosphere without burningor sintering. The polyurethane foams described herein have been found tobe particularly resistant to sintering, even when repeatedly subjectedto gas ignition within a headspace. Finally, the materials must haveadequate chemical stability against dissolution or other degradationwithin the cells. As mentioned previously, polypropylene is known to behighly resistant to dissolution in both aqueous acid and alkalineelectrolytes. The polyurethane foams have excellent stability in acidelectrolyte, but not in highly alkaline solutions. Storage of thesematerials in a sulfuric acid electrolyte at 160° F. (71° C.) for morethan two months resulted in no physical degradation of the foam and nodetection of appreciable dissolved organic material in the electrolyte.Extended field tests in automotive batteries operated under a variety ofconditions also resulted in no dissolution or degradation of thematerial.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim:
 1. In an electric storage cell including a substantiallyclosed container, electrode elements and an electrolyte disposed withinthe container, a normally open head space within the container above theelements and electrolyte, and vent means in the container for therelease of gases generated by electrochemical reactions within the cell,an improved material for insertion in the head space to attenuate anexplosion within the cell as a result of ignition of the gases,comprising a porous compressible plastic material having a bimodal poredistribution including a major proportion of small pores effective tolimit the pressure build-up resulting from gas ignition and a minorproportion of large pores sufficient to accommodate movement of thegases or electrolyte through the head space occurring during celloperation.
 2. The invention as defined in claim 1 wherein the porouscompressible plastic material comprises randomly oriented pieces of opencell foam substantially filling the head space.
 3. The invention asdefined in claim 2 wherein the open cell foam material is polyurethane.4. The invention as defined in claim 3 wherein the proportion of smallpores is provided in the open cells of the foam pieces and theproportion of large pores is provided in the spaces between the randomlyoriented pieces.
 5. The invention as defined in claim 4 wherein thematerial pieces are of uniform size and substantially cube-shaped. 6.The invention as defined in claim 3 wherein the material comprises acomposite of first pieces having substantially uniform small pores andsecond pieces having substantially uniform large pores.
 7. The inventionas defined in claim 6 wherein the material comprising the first pieceshas a pore size greater than 50 pores per inch and the materialcomprising the second pieces has a pore size less than 50 pores perinch.
 8. The invention as defined in claim 6 wherein the materialcomprising the first pieces has a pore size of 60 pores per inch and thematerial comprising the second pieces has a pore size of 20 pores perinch.
 9. The invention as defined in claim 8 wherein the volume ratio offirst material pieces to second material pieces is about 3:1.
 10. Theinvention as defined in claim 9 wherein the composite material ismaintained in compression within the head space.
 11. The invention asdefined in claim 10 wherein the composite material is maintained incompression ranging from 80 to 95% of its free volume.
 12. The inventionas defined in claim 1 wherein the porous compressible plastic materialcomprises randomly oriented pieces of bulked non-woven fiberssubstantially filling the head space.
 13. The invention as defined inclaim 1 wherein the fibers are made of a polyolefin.
 14. The inventionas defined in claim 13 wherein the proportion of small pores is providedwithin the material and the proportion of large pores is provided in thespaces between the randomly oriented pieces.
 15. The invention asdefined in claim 14 wherein the polyolefin fibers comprisepolypropylene.
 16. The invention as defined in claim 15 wherein thematerial is retained in the head space in a substantially uncompressedstate.
 17. The invention as defined in claim 1 wherein the electrolyteis substantially immobilized within the electrode elements and theproportion of large pores in the plastic material is essentially zero.18. In an electric storage cell including a substantially closedcontainer, electrode elements and an electrolyte disposed within thecontainer, vent means in the container for release of gases generated bythe electrochemical reaction within the cell, and a normally open headspace within the container above the elements and electrolyte, themethod of attenuating an explosion within the cell resulting from theignition of the gases generated comprising the steps of:(1) preparing aporous compressible plastic material having a distribution of a majorproportion of small pores effective to limit the pressure build-upresulting from gas ignition, and a minor proportion of large poressufficient to accommodate movement of the gases or electrolyte throughthe head space occurring during cell operation; and, (2) substantiallyfilling the head space of the cell with the material.
 19. The method asdefined in claim 18 wherein the plastic material is selected from thegroup comprising polyolefins, polyurethanes and polyesters.
 20. Themethod as defined in claim 18 wherein the plastic material comprisesrandomly oriented pieces of open cell polyurethane foam.
 21. The methodas defined in claim 19 wherein the plastic material comprises randomlyoriented pieces of bulked non-woven fibers.
 22. The method as defined inclaim 21 wherein the fiber material is polypropylene.